1. Field of the Invention
This invention relates to field effect transistors (FETs) and in particular to power FETs of the n-channel type, and especially to those made of group III-V compounds, such as gallium arsenide MESFETs and aluminum gallium arsenide/gallium arsenide HEMTs, having an improved structure which permits them to operate more efficiently at high voltages at RF frequencies.
2. Background Information
An important requirement in many system applications, including phased array radars is a FET device that can operate at large drain bias voltages and deliver radio frequency (RF) power with high gain and good conversion efficiency. A device commonly used for these applications is the metal-semiconductor field effect transistor (MESFET) and typically such devices made of gallium arsenide (GaAs). High gate-drain breakdown voltages can be readily achieved in MESFET structures, even in devices with high doping levels, by carefully controlling the amount of undepleted donor charges in the gate-drain region. However, it has been observed by me that such devices with high breakdown voltages generally exhibit poor RF performance and show premature saturation. A similar observation has been made by others who have noticed that unrecessed or shallow recessed devices yield a higher breakdown voltage but lower output power and efficiency than deeper recessed devices. This behavior was attributed to surface effects, but no explanation of the physical mechanisms involved was given. Other works have also observed premature power saturation in GaAs MESFET devices and attributed it to either bulk or surface effects.
Additional research has shown that deep levels in the buffer layer-active layer interface were responsible for premature power saturation in GaAs MESFETs. This research also showed that for materials grown under proper conditions, the deep level concentrations were very low, and there was no early power saturation. With the advances that have taken place in epitaxial growth techniques, high purity GaAs material can easily be obtained by using molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), or vapor phase epitaxy (VPE). In these materials, the residual defect concentrations are very low and there is high electron mobility. Hence, bulk deep level effects are expected to be small. However, even in such high purity materials, the surface effects could pose a problem. For example, others have correlated power saturation in MESFETs with transconductance dispersion which in turn was related to surface effects. It was explained that the observed dispersion in transconductance arose due to a surface conducting layer which behaved like a high valued resistor. Though the physical origin of this surface layer is not very clear, it is believed to be associated with free arsenic at the surface. The surface mechanism of GaAs has also been believed to be responsible for the degradation of power MESFET performance during reliability tests. Unlike the bulk trapping problem, it is very difficult to solve the surface problem in GaAs since at the present there are no known ways of reliably passivating the surface of GaAs.
Similar surface effect problems are encountered in high electron mobility transistor (HEMTs) made of GaAs and AlGaAs, and it is believed would be present in MESFETs and HEMTs made of other group III-V compounds of the periodic table.
There is a need therefore for FETs which can operate at high voltgates at RF power with good efficiency.